Regenerative Endodontic Procedure in an Immature Permanent Incisor with Internal Root Resorption: a Case Report

This report describes a regenerative endodontic procedure of an immature permanent incisor with internal root resorption (IRR) and 4-years follow-up. A healthy 8-year-old man was referred for treatment of tooth #9 after a traumatic intrusion. The periapical radiograph showed an IRR and an open apex with periradicular lesion. A diagnosis of pulp necrosis and chronic apical abscess was achieved. In the first appointment, under local anesthesia and rubber dam isolation, an access cavity was designed and the root canal was chemically cleaned under irrigation with 10 mL 1.5% sodium hypochlorite (NaOCl). The root canal was then dried and calcium hydroxide paste was placed. During the second appointment, the root canal was irrigated with 5 mL of 17% ethylenediaminetetraacetic acid (EDTA) for 5 minutes and dried. The blood clot was established in a time of 3 minutes after the bleeding from the periapical tissue was trigged. White mineral trioxide aggregate (MTA) was placed up to the amelocemental junction and the final restoration of the access cavity was carried out. During periodic clinical and radiographic follow-up, the patient remained symptom free, the periapical region was completely healed, inhibition of the root resorption process achieved, and formation of the new periodontal ligament as well as tooth widening development observed, meeting functional expectations after 48 months. The regenerative endodontic procedures are an available option to treat IRR in severely immature teeth. The available literature on the regenerative endodontic procedures applied to IRR treatment is limited, and more research is needed in this field.

(NaOCl). The root canal was then dried and calcium hydroxide paste was placed. During the second appointment, the root canal was irrigated with 5 mL of 17% ethylenediaminetetraacetic acid (EDTA) for 5 minutes and dried. The blood clot was established in a time of 3 minutes after the bleeding from the periapical tissue was trigged. White mineral trioxide aggregate (MTA) was placed up to the amelocemental junction and the final restoration of the access cavity was carried out. During periodic clinical and radiographic follow-up, the patient remained symptom free, the periapical region was completely healed, inhibition of the root resorption process achieved, and formation of the new periodontal ligament as well as tooth widening development observed, meeting functional expectations after 48 months. The regenerative endodontic procedures are an available option to treat IRR in severely immature teeth. The available literature on the regenerative endodontic procedures applied to IRR treatment is limited, and more research is needed in this field.

Introduction
Root resorption occurs due to the loss of dental hard tissues as a result of the action of odontoclasts [1] and might be classified into external or internal resorption depending on the location of the lesion in relation to the root surface [2]. Internal root resorption (IRR), being the less commonly occurring type of root resorption [3], is a resorptive defect of the internal aspect of the root [4]. The image of this lesion is normally round to oval and continues within the canal space.
Its pathogenesis occurs because of the activity of odontoclasts, which are multinucleated cells able to form resorption lacunae [4]. For these cells can perform their function, thus, for IRR to occur, the protective odontoblasts layer and predentine surrounding the canal wall should be pre-damaged originating the exposure of the underlying mineralized dentin to odontoclasts [5].
Without bacterial stimulation, the resorption will be self-limited. However, if there is a chronic stimulation through the infected necrotic coronal tissue, and a viable blood supply from the pulp tissue apical to the resorptive lesion, those clastic cells will continue in time with their function [2], being able to produce a perforation with the periodontal ligament [6], worsening the prognosis and hindering the treatment. Traditionally, due to the idiosyncrasy of IRR, root canal treatment has been the treatment of choice and requires chemo-mechanical preparation and root canal obturation with thermoplastic gutta-percha techniques [3].
In some cases, a surgical approach is indicated, predominantly when the area of resorption cannot be accessed through the canal or has progressed through the tooth and has reached the periodontium [7].
The foundation of "revascularization" and "regenerative endodontic treatment" (RET) terms was started in the 60´s [8]. RET allows the regeneration of the root by reestablishing blood flow to the tooth in cases of pulp necrosis in permanent teeth with open apex, not only eliminating and/or preventing apical periodontitis, but the promotion of root development, and hence preventing root fracture [9]. The most recent innovation in the treatment of IRR is RET since this option provides the opportunity for replacement of missing structure caused by the resorption and thus, giving a better prognosis in the long-term for those teeth affected [4,10]. Nevertheless, RET for IRR is a technique that, to our knowledge, has only be applied in teeth with completed developed roots. Therefore, the aim of this case report is to present a RET procedure of a perforated IRR in a tooth with open apex and its clinical and radiographic findings after 48 months.    [14].

Case Presentation
On the other hand, the classical treatment for IRR has been conventional root canal treatment [3]. Whereas traditional root canal treatment is aimed to seal the root canal system with biocompatible materials and the objective of preventing reinfection, RET is intended to reestablish vitality, immunity and if possible, sensitivity of the pulp tissue. This is an important issue as only vital tissue has the capacity to repair, regenerate, and generates immune defense response. In fact, RET can be applied in order to improve the prognosis by increasing the length and promoting the root development [10,14] This may also be accepted for improving the prognosis of teeth affected by IRR as this treatment option is able to provide the replacement of missing dental tissues produced by the resorptive lesion [4,10].
Based on the classification of Cveck [15], our case featured a stage 2 since only half of the root was developed at the moment of the intervention. Following the "clinical considerations to perform a regenerative procedure" given by the American Association of Endodontics, it is indicated to accomplish a RET in order to allow healing and root development [16].
Taken into consideration that necrotic and infected pulp tissue in IRR must be removed [3,17] and the benefits of intracanal medicaments [13], calcium hydroxide was used in this case in a two-visit treatment plan. Nevertheless, calcium hydroxide must be used limited in time as it can weaken the fragile root structure and increase the risk of root fracture [18].
During treatment, mechanical preparation was avoided in order to preserve the already weaken root structure. At the same time, as previously described, a gentle chemical disinfection of the root canal by using 1.5% NaOCl was carried out [19][20]. Though higher NaOCl concentration (e.g. 3% or 6%) should probably have greater antimicrobial capability [21], at the same time, a higher concentration would be more cytotoxic for periodontal ligament cells and stem cells from apical papilla (SCAPs) responsible for RET [20,22]. However, as elimination of any remainder granulation tissue present in the resorptive lesion and necrotic tissue is essential for the success of the treatment, calcium hydroxide was used as medication between appointments [23]. The most commonly used medications for RET are based on the combination of three antibiotics (ciprofloxacin, metronidazole, and minocycline) [14] or two (ciprofloxacin and metronidazole) [24]. In the present case, calcium hydroxide was chosen as in previous studies [25] because of its optimal antimicrobial capability to create a basic medium able to neutralize osteoclastic activity [26] and its ability to promote the proliferation of the stem cells from the dental papilla [27]. Moreover, according to the European Endodontic Society and the American Association of Endodontists, the use of calcium hydroxide can provide a similar outcome rate as the one produced by an antibiotic paste [28]. In addition, the possibility of discoloration, bacterial resistance, and allergic reaction generated by the combination of these drugs will be reduced [29].
The 17% EDTA was used as exclusive irrigant solution during the second appointment in order to remove the calcium hydroxide previously placed and at the same time, promoting environmental conditions that stimulate the survival of SCAPs needed in RET [13,19,20] as it stimulate to release growth factors present into the dentin matrix [30]. Growth factors such as TGF-β1, fibroblast growth factors 2 or vascular endothelial growth factors are essential as they determine the fate of stem cells and help in tissue engineering [13].
The induction of intra-canal bleeding was carried out with a #30 K file to willfully develop periapical tissue bleeding into the root canal with the aim to provide a blood scaffold. At the same time, platelet-derived growth factors and mesenchymal stem cells will be introduced into the root canal for pulp tissue reparation [13]. A biocompatible material must be placed between the clot and the final restorative material. Although calcium hydroxide and its derivatives have been described in the literature for this aim, MTA was used in this case as it is one of the most extended material used for this purpose due to its high biocompatibility, essential for the treatment outcome [31].
Two publications reported discoloration after RET in 40% of cases [32][33], as in this case occurred. Although discoloration is more likely to occur with triple antibiotic paste which includes minocycline, it has also been reported with calcium hydroxide and MTA as intracoronal barrier [34]. In this case, the removal of MTA and a buccal direct composite veneer resulted in a satisfactory aesthetic solution for the patient and tutor.
Furthermore, recall examination for at least 36 months is essential for the radiographic assessment of apical healing and root development [35]. In this case, a follow up to 48 months was carried out.
Nevertheless, some limitations were present in this clinical report and must be taken into consideration. al. [36] in a clinical study of trau-matized immature teeth with necrotic pulps, this case neither demonstrated continuation of root development. However, apical closure and periapical healing was obs-erved. Given that the patient was a child, the legal guar-dian signed an informed consent in which the advantag-es, limitations of this treatment option, consequences of its failure, the alternative therapeutic options, and steps to be followed in case of side effects were noted.

Conclusion
RET is an alternative modality to treat severely traumatized teeth ongoing the appropriate therapeutic protocols. Future clinical studies should analyze clinical success rate for RET in immature teeth with IRR.